Atmospheric Pressure and the Bends

By Joan Whetzel

Atmospheric Pressure measures the weight of the atmosphere on the Earth, which can vary hourly or daily. The changes in pressure can affect our bodies to varying degrees, but it has the greatest effect on those who deal in rapid or significant changes in elevation or depth, and with it the associated changes in atmospheric pressure, namely SCUBA divers, fighter pilots and astronauts. The lack of proper precautions could cause the bends, otherwise known as decompression sickness.


The Effects of Atmospheric Pressure

Changes in altitude above sea level and depths below sea level coincide with changes in atmospheric pressure. At sea level the average pressure is 14.70 pounds per square inch (psi) or 1013.25 millibars (mb), at 1,000 feet below sea level the pressure runs 15.23 psi (1048.15mb), and at 1,000 feet above sea level, there is a pressure rating of 14.16 psi (977.26). This may not sound like much of difference, but it makes a big difference in our bodies. The higher pressures added to the body below sea level cause the air bubbles in a diver's tissues and bloodstream to compress. These air bubbles expand when ascending back to sea level. For fighter pilots and astronauts, the air bubbles expand as they rise higher in the atmosphere also. In all of these cases, if the rising in depth or elevation occurs to quickly, it has the same effect as opening a soda bottle; the air bubbles expand rapidly causing a kind of fizzing in the blood and tissues. If the air bubbles collect as one big bubble in the blood stream, it becomes a deadly air embolism. If the bubbles collect in the tissue and joints, it causes them to stiffen, thereby giving the person a case of the bends.

Atmospheric Pressure and SCUBA Diving

SCUBA Divers suffer the weight of both the air and the water, causing the air in the blood and tissues to compress. Additionally, compressed air is used for breathing while diving. When the dive is completed, and the diver begins ascending to the surface, the pressure outside the diver’s body is lifted, allowing the compressed air inside the diver’s body to expand. All that expanding air has to go somewhere.

Divers take there time resurfacing from a dive, taking periodic stops on the way up to expel the expanding air through exhalation. If these stops were not taken, and the extra air was not expelled, it could have serious consequences on the diver’s body. First, the air would expand in the diver’s lungs to the point that they could explode because they cannot hold that much air under at sea level or higher altitudes. Second, the air bubbles (nitrogen mainly) collect in tissue and joints, and continue to expand, causing them to stiffen into a condition called the bends. If not corrected quickly the damage could become permanent.

Finally, if a diver rises too quickly, the air bubbles release from the tissues into the blood and expand too quickly, making the blood frothy. This event is often compared to opening a soda bottle under pressure, and the soda bubbles suddenly erupting within the soda. If the air bubbles collect into one large bubble, they become an air embolism that travels to the lungs or brain, which is lethal..

Divers are warned not to go mountain climbing or flying in airplanes for several hours after a dive. Even though their ascension to the surface was safe, allowing adequate time to surface without any signs of decompression sickness, there is still a residual buildup of nitrogen in the tissues and bloodstream for a few hours afterward. The rapid rise in altitude too soon after a dive, could bring on a case of the bends.

Decompression Sickness and Fighter Pilots

When the Navy began sending its fighter jets to higher altitudes to test new equipment, the pilots began experiencing decompression sickness due to the rapid change in altitudes (which meant drastic changes in the atmospheric pressure and the expansion of air bubbles in the tissues and bloodstream) and also due to the lack of pressurized cabins. The Navy was spurred into creating pressure suits for their fighter pilots to use while on these missions and test flights at such extreme altitudes. Fighter pilots were also given 100% Oxygen treatments for 30 minutes prior to and during flights. Without these precautions, the fighter pilots suffered the same symptoms as divers who ascend to quickly.

Decompression Sickness and Astronauts

Space is considered a near vacuum, meaning there is little or no air pressure weighing on the human body when out in space. Astronauts use pressurized suits during take offs and landings, specialty pressurized space suits for Extra Vehicular Activities (EVA), pressurized cabins in the space ships, and pressurized living quarters in the space station. Without them, humans would not be able to survive longer than about 15 seconds in space. Without them, the nitrogen bubbles cause the tissues and joints to freeze up (so to speak) and produce so many bubbles in the bloodstream that the fizzing and frothing become almost like a boiling sensation, like shaking up that soda bottle before releasing the pressure. The “boiling” blood expands to the point of blowing out the capillaries and blood vessels. Astronauts also receive 100% oxygen therapy for 3 hours before and during a space walk, because their EVA suits can only sustain a pressure equivalent to about 35% of the pressure found at sea level. By filling their tissues and bloodstream with oxygen, it prevents the build up of nitrogen, thereby preventing decompression sickness upon returning to the pressurized space ship cabin or the space station.

Theprecautions taken by divers, fighter pilots, astronauts, and commercial airlines are intended to equalize the pressure inside and outside of peoples’ bodies. This keeps the nitrogen from building up, avoiding the bends.

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